More than one way down

Posted on December 15, 2009

The influence of a top predator often cascades down the food chain: predators at the top feeding level deplete prey populations at the next level down, which relaxes predation pressure on populations at the level below that, and so on.  Most research on top-down cascades has focussed on food chains made up of predators, herbivores and living plants, with much less attention being devoted to the other main type of food chain, comprised of predators, detritivores and detritus (decomposing organic matter).  Field experiments in an English chalk stream provided evidence for a cascade effect, with a fish species (bullhead, Cottus gobio) as the top predator and a shrimp (Gammarus pulex) as the main detritivore.  When bullhead were present, shrimp densities and rates of detritus processing fell.  Bullhead predation also affected the herbivorous food chain by reducing the densities of grazing snails, but there was no cascading impact on algal production, which appeared to be controlled more by bottom-up processes.  The results emphasise the  need to consider both types of energy pathway when trying to understand food web dynamics.

Reference: Woodward,G., Papantoniou, G., Edwards, F. & Lauridsen, R.B.  2008.  Trophic trickles and cascades in a complex food web: impacts of a keystone predator on stream community structure and ecosystem processes.  Oikos 117,  683-692.